1. Field of the Invention
This invention relates generally to high-speed signal delivery, and, more particularly, to techniques for compensating for distortion suffered by high-speed digital signals transmitted through band-limited channels.
2. Description of Related Art
With the advent of high-speed technologies such as SerDes and SONET, which operate at speeds above 1 GBit/s, the interconnections between devices have assumed an important role in limiting overall system speed. Transmission paths between devices or portions of electronic systems impose losses. These losses can attenuate digital signals to a point where they cannot reliably be received. They can also induce timing variations, or jitter.
FIG. 1 shows a conventional circuit for transmitting and receiving a digital bit stream, here a serial bit stream. A memory 110 stores N-bit digital data. The N-bit data is clocked out of the memory 110, and a serializer 112 converts the N-bit data into a serial bit stream “SIG IN” having a bit rate that is N times the memory clocking rate. A driver 114 buffers the serial bit stream to generate “SIG TR,” and sends SIG TR down a transmission path 116. The transmission path generally attenuates or otherwise distorts SIG TR, so that the output “SIG REC” of the transmission path 116 is a distorted version of SIG TR. A comparator 118 receives SIG REC and compares it with a threshold level to generate an output signal, “SIG OUT.”
As shown in FIG. 2, SIG OUT may not accurately represent SIG IN, depending on the losses in the transmission path 116 and the speed of signals transmitted. Short pulses may be attenuated to a point where they are not received at all. Assuming they are received, they may be significantly shortened in width and are subject to increased jitter. Longer pulses also suffer from timing errors, because they are delayed by the distorting characteristics of the transmission path 116. In addition, pulsewidths of both long and short pulses are distorted, owing to differences between risetimes and falltimes of signals passed through the transmission path.
We have recognized, as shown in FIG. 3, that the distortion imposed by the transmission path 116 can be described by a transfer function H(s). To pre-compensate for the distortion induced by H(s), we observe that one need only pass the bit stream through a filter 310 having a transfer function H−1(s). The effects of the filter 310 then balance the distortion of the transmission path 116, to produce a receiver signal SIG REC that accurately resembles SIG IN.
Realizing this type of filter tends to be difficult in practice, however. Analog filters for high frequency signals tend to be physically large. In addition, optimal characteristics are often difficult to establish or change to accommodate different transmission paths. Although they are usually smaller and more flexible, digital filters present drawbacks as well. Because they involve mathematical operations, such as addition and multiplication, they tend to be ill suited for extremely fast signals that change faster than a digital filter's mathematical circuits can handle.
What would be desirable is a compensation technique that could be realized in a small area, could be readily adapted for different transmission paths, and could be operated on extremely fast digital bit streams.